Just like a toothbrush, the best products are used every day - sometimes multiple times per day. There are often no other options to solve the problem at hand. And the problem is prevalent for everybody.

As you develop your product, does it pass the Toothbrush Test?

What problem does it solve?

How often does this problem occur?

Will people be reminded of it every day?

Can a habit of use be established with it?

Are there other products that solve the problem in a different way?

As soon as you can convince your customers that they need your product, you've won.